DOCSLIB.ORG

1 Thermally-Driven Atmospheric Escape: Transition From

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
1 Thermally-Driven Atmospheric Escape: Transition From Thermally-driven atmospheric escape: Transition from hydrodynamic to Jeans escape Alexey N. Volkov1, Robert E. Johnson1,2, Orenthal J. Tucker1, Justin T. Erwin1 1Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904-4745 2Physics Department, NYU, NY, NY 10003-6621 Abstract Thermally-driven atmospheric escape evolves from an organized outflow (hydrodynamic escape) to escape on a molecule by molecules basis (Jeans escape) with increasing Jeans parameter, the ratio of the gravitational to thermal energy of molecules in a planet’s atmosphere. This transition is described here using the direct simulation Monte Carlo method for a single component spherically symmetric atmosphere. When the heating is predominantly below the lower boundary of the simulation region, R0, and well below the exobase, this transition is shown to occur over a surprisingly narrow range of Jeans parameters evaluated at R0: λ0 ~ 2-3. The Jeans parameter λ0 ~ 2.1 roughly corresponds to the upper limit for isentropic, supersonic outflow and for λ0 >3 escape occurs on a molecule by molecule basis. For λ0 > ~6, it is shown that the escape rate does not deviate significantly from the familiar Jeans rate evaluated at the nominal exobase, contrary to what has been suggested. Scaling by the Jeans parameter and the Knudsen number, escape calculations for Pluto and an early Earth’s atmosphere are evaluated, and the results presented here can be applied to thermally-induced escape from a number of solar and extrasolar planetary bodies. 1 Introduction Our understanding of atmospheric evolution is being enormously enhanced by extensive spacecraft and telescopic data on the outer solar system bodies and on exoplanets. The large amount of data on Titan’s atmosphere from the Cassini spacecraft led to the use of models for atmospheric escape that predicted rates that differed enormously (Johnson 2009; Johnson et al. 2009). This disagreement was due to a lack of a kinetic model for how atmospheric escape changes in character from evaporation on a molecule by molecule basis to an organized flow, referred to as hydrodynamic escape, a process of considerable interest early in the early stages of the evolution of a planet’s atmosphere (e.g., Watson et al. 1981; Hunten 1982; Tian et al. 2008) and to the evolution of exoplanet atmospheres (e.g., Murray-Clay et al. 2009). The transition from hydrodynamic to Jeans escape is described in terms of the local Jeans parameter, λ = U(r)/kT(r), where U(r) is a molecule’s gravitational energy at distance r from the planet’s center, T is the local temperature, and k is the Boltzmann constant (e.g., Chamberlain and Hunten 1987; Johnson et al. 2008). In Hunten (1982) it was suggested that if λ decreases to ~2 above the exobase, then the escape rate is not too different from the Jeans rate, but if it becomes ~2 near or below the exobase, hydrodynamic escape can occur. Since it was assumed that the transition region from Jeans to hydrodynamic escape occurred over a broad range of λ, an intermediate model, referred to as the slow hydrodynamic escape (SHE) model, was developed to describe outflow from atmospheres such as Pluto’s with exobase values λ~10 (Hunten and Watson 1982; McNutt 1989; Krasnopolsky 1999; Tian and Toon 2005; Strobel 2008a). In this model, based on that of Parker (1964) for the solar wind, the continuum fluid equations accounting for heat conduction are solved to an altitude where the flow velocity is still smaller than the speed of sound, and then asymptotic conditions on the temperature and density 2 are applied. Unfortunately, this altitude is often above the nominal exobase. This model, subsequently applied to even Titan (λ~40), could overestimate the mass loss rate (Tucker and Johnson 2009; Johnson 2010). Other recent models simply couple the Jeans rate to continuum models at the exobase, even when the escape rates are quite large (Chassefiere1996; Tian et al. 2009), or they couple to a modified Jeans rate (Yelle 2004; Tian et al. 2008). Here the transition from Jeans to hydrodynamic escape from a planetary body is described via a kinetic model. Escape from an atmosphere can be calculated from the Boltzmann equation, which can describe both continuum flow and rarefied flow (e.g., Chapman and Cowling 1970) at large distances from a planet. Here we use the Direct Simulation Monte Carlo (DSMC) method (e.g. Bird 1994) to simulate thermally-driven flow in a one-dimensional radial atmosphere. For escape driven by heat deposited below the lower boundary of the simulation region, R0, we show that the transition occurs over a surprisingly narrow range of Jeans parameters evaluated at R0 (λ0 ~2 – 3). That is, hypersonic outflow drives escape for λ0 ≤ ~ 2, but above λ0 ~ 3 hypersonic flow does not occur, even at large distances from the exobase, and for λ0 > ~6 the escape rate is close to the Jeans escape rate. Following the description of the DSMC model, results for a range of λ0 are presented with emphasis on the transition region. 2. DSMC simulations A kinetic model for calculations of the structure of the upper atmosphere and the escape rate is based on the Boltzmann kinetic equation. A DSMC method (Bird 1994), which is a stochastic method for numerical solution of problems based on the Boltzmann equation, is used here to simulate a spherically-symmetric, single component atmosphere supplied by out gassing from a surface at radius R0 . This can be the actual surface with a vapor pressure determined by the solar insolation or a virtual surface in the atmosphere, above which little additional heat is 3 deposited and at which the density and temperature are known. In such a model, escape is driven by thermal conduction and heat flow from below R0 . In DSMC simulations, the real gas is simulated by means of large number of representative molecules of mass m. Trajectories of these molecules are calculated in a gravity field and subject to mutual collisions. It is readily shown that for a velocity independent cross section, the Boltzmann equations, and, hence, the results presented here, can be scaled by two parameters: the source values of the Jeans parameter, λ0, 1/2 -1 and a Knudsen number, Kn0 = l0 /R0 where l0 =(2 n0σ) is the mean free path of molecules at the lower boundary with σ the collision cross section and n0 the number density on lower boundary. The Knudsen number often discussed for a planet’s atmosphere is Kn(r) = l / H, where l and H are the local mean free path of molecules and the atmospheric scale height at radial distance r. Since Kn(R0) = λ0 Kn0, it could be used, instead of Kn0, as one of the two scaling parameters. Simulations were conducted on a non-homogeneous mesh and the number of simulated molecules in a cell was varied over a wide range (e.g., Volkov et al. 2010). Results are presented for the hard sphere collisions, but comparisons made using variable hard spheres and forward directed collision models (e.g.,Tucker and Johnson 2009) gave similar results. In the kinetic model, the flow at each r is described in terms of a velocity distribution function, f (r,v||,v⊥ ,t) , where v|| and v⊥ are velocity components parallel and perpendicular to radial direction and t is time. Macroscopic parameters are calculated for molecular quantities Ψ(r,v|| ,v⊥ ) using the +∞∞ integral operator Ψ = 2π Ψ(r,v ,v ) f (r,v ,v ,t)v dv dv , e.g., number density, n ∫∫ || ⊥ || ⊥ ⊥ ⊥ || −∞ 0 2 ( Ψ = 1), radial flow velocity, u (Ψ = v|| / n), parallel, T|| (Ψ = m(v|| − u) /(nk) , and 4 2 perpendicular, T⊥ ()Ψ = mv⊥ /(2nk ), temperatures with T = (T|| +2T⊥ )/3. The molecules have gravitational energyU (r) = −GMm / r , where M is the planet’s mass and G is the gravitational constant. The mass of the gas above R0 is assumed to be much smaller than M so that self- gravity is neglected. The velocity distribution in the boundary cell at r=R0 is maintained to be Maxwellian at a fixed n0 and T0 , and zero gas velocity. The exit boundary at r = R1 is placed far enough from R0 so the gas flow above this boundary can be approximately treated as collisionless. A molecule crossing R1 with velocities v|| and v⊥ will escape if 1/ 2 2 2 v > (−2U (R1) / m) , where v = v|| + v⊥ , while a molecule with a smaller v will return to R1 with − v|| and v⊥ . Since collisions can modify the flow even at relatively large r, the effect of the position of R1 was studied (Volkov et al. 2010) and R1 is chosen to be sufficiently large in order to eliminate the effect of upper boundary in the flow region described. The simulations were carried out using the following fixed parameters, m, σ, T0 and R0 with λ0 and Kn0 varied by changing M and n0. However, the results scale with λ0 and Kn0 in a sense that any two flows with different m, σ, T0, R0, M, and n0 represent the same flow in a dimensionless form, if the λ0 and Kn0 are the same. Simulations are initiated by ‘evaporation’ from the cell at R0 until steady flow is reached at large t. Steady-state flow is assumed to have occurred when the number flux 4πr 2n(r)u(r) = Φ varies by less that ~1% across the domain, where Φ is the escape rate. The Jeans escape rate, ΦJeans, is defined by the upward flux of molecules with speeds 1/ 2 v ≥ (−2U (r) / m) at the nominal exobase rexo, typically determined by the scale height [l(rexo) = H(rexo)] at large λ0 or by the curvature [l(rexo) = rexo] at small λ0.
Load more

Recommended publications
  • Modeling the Formation of the Earth's Atmosphere by Hydrodynamic
    Origin of the Earth and Moon Conference 4102.pdf MODELING THE FORMATION OF THE EARTH’S ATMOSPHERE BY HYDRODYNAMIC ESCAPE AND PLANETARY OUTGASSING. R. O. Pepin, School of Physics and Astronomy, University of Minnesota, Minneapolis MN 55455, USA. Accretion of lunar- to Mars-sized terrestrial Models of this kind have had some success in planet embryos is believed to have occurred on accounting for the details of terrestrial noble gas timescales of »105 years in the presence of nebular mass distributions [3,6]. However, they are not with- gas [1,2]. Mechanisms for trapping nebular (“solar”) out problems. “Solar” isotopic distributions in the noble gases in these embryos include occlusion of initial terrestrial reservoirs are taken to be those ambient gases in the planetesimals accreted to form measured in the solar wind. Although current esti- them, and, probably more important, efficient ad- mates for the isotopic compositions of solar-wind Ne, sorption of gravitationally condensed nebular gases Ar, and Kr are compatible with those required in the on embryo surfaces once they had grown to modeling for Earth’s primordial noble gas invento- ~Mercury size [3]. During the following ~100–200 ries, this assumption that the wind correctly repre- m.y. of growth through the “giant-impact” stage sents the composition of the nebular source supply- [1,4] to a fully assembled planet, a coaccreting pri- ing these gases to the early Earth is not strictly valid mordial atmosphere is likely to develop by impact- for Xe. Generating the isotope ratios of terrestrial degassing of colliding embryos and inward-scattered nonradiogenic Xe by fractionation in hydrodynamic icy planetesimals, and by gravitational capture of escape requires an initial composition called U-Xe, nebular gases if the gas phase of the nebula had not which appears to be isotopically identical to meas- yet fully dissipated.
    [Show full text]
  • The Chemical Origin of Behavior Is Rooted in Abiogenesis
    Life 2012, 2, 313-322; doi:10.3390/life2040313 OPEN ACCESS life ISSN 2075-1729 www.mdpi.com/journal/life Article The Chemical Origin of Behavior is Rooted in Abiogenesis Brian C. Larson, R. Paul Jensen and Niles Lehman * Department of Chemistry, Portland State University, P.O. Box 751, Portland, OR 97207, USA; E-Mails: [email protected] (B.C.L.); [email protected] (R.P.J.) * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +1-503-725-8769. Received: 6 August 2012; in revised form: 29 September 2012 / Accepted: 2 November 2012 / Published: 7 November 2012 Abstract: We describe the initial realization of behavior in the biosphere, which we term behavioral chemistry. If molecules are complex enough to attain a stochastic element to their structural conformation in such as a way as to radically affect their function in a biological (evolvable) setting, then they have the capacity to behave. This circumstance is described here as behavioral chemistry, unique in its definition from the colloquial chemical behavior. This transition between chemical behavior and behavioral chemistry need be explicit when discussing the root cause of behavior, which itself lies squarely at the origins of life and is the foundation of choice. RNA polymers of sufficient length meet the criteria for behavioral chemistry and therefore are capable of making a choice. Keywords: behavior; chemistry; free will; RNA; origins of life 1. Introduction Behavior is an integral feature of life. Behavior is manifest when a choice is possible, and a living entity responds to its environment in one of multiple possible ways.
    [Show full text]
  • Water on Venus: Implications of Theearly Hydrodynamic Escape
    EPSC Abstracts Vol. 5, EPSC2010-288, 2010 European Planetary Science Congress 2010 c Author(s) 2010 Water on Venus: Implications of theEarly Hydrodynamic Escape C. Gillmann (1), E. Chassefière (2) and P. Lognonné (1) (1) Institut de Physique du Globe (IPGP), Paris, France, ([email protected]) (2) CNRS/UPS UMR 8148 IDES Interactions et Dynamique des Environnements de Surface, Paris, France Abstract toward a common origin for those three atmospheres and a usual theory is that these atmospheres are In order to study the evolution of the primitive secondary, created by the degassing of volatiles from atmosphere of Venus, we developed a time the bodies that constituted the early planet. The dependent model of hydrogen hydrodynamic escape atmosphere of Venus could then represent a primitive state of the evolution of terrestrial. Moreover, Mars powered by solar EUV (Extreme UV) flux and solar and the Earth possess reservoirs of water at present- wind, and accounting for oxygen frictional escape day whereas Venus seems to be dry. The early We study specifically the isotopic fractionation of evolution of terrestrial planets and the effects of noble gases resulting from hydrodynamic escape. hydrodynamic escape might explain this observation The fractionation’s primary cause is the effect of by the removal of most of the initial water on Venus. diffusive/gravitational separation between the homopause and the base of the escape. Heavy noble 2. Results and Scenario gases such as Kr and Xe are not fractionated. Ar is only marginally fractionated whereas Ne is We study the evolution of the primitive atmosphere moderately fractionated. of Venus and investigate the possibility of an early We also take into account oxygen dragged off habitable Venus with a possible liquid water ocean on along with hydrogen by hydrodynamic process.
    [Show full text]
  • Cosmochemistry Cosmic Background RadiaOn
    6/10/13 Cosmochemistry Cosmic background radiaon Dust Anja C. Andersen Niels Bohr Instute University of Copenhagen hp://www.dark-cosmology.dk/~anja Hauser & Dwek 2001 Molecule formation on dust grains Multiwavelenght MW 1 6/10/13 Gas-phase element depleons in the Concept of dust depleon interstellar medium The depleon of an element X in the ISM is defined in terms of (a logarithm of) its reducon factor below the expected abundance relave to that of hydrogen if all of the atoms were in the gas phase, [Xgas/H] = log{N(X)/N(H)} − log(X/H) which is based on the assumpon that solar abundances (X/H)are good reference values that truly reflect the underlying total abundances. In this formula, N(X) is the column density of element X and N(H) represents the column density of hydrogen in both atomic and molecular form, i.e., N(HI) + 2N(H2). The missing atoms of element X are presumed to be locked up in solids within dust grains or large molecules that are difficult to idenfy spectroscopically, with fraconal amounts (again relave to H) given by [Xgas/H] (Xdust/H) = (X/H)(1 − 10 ). Jenkins 2009 Jenkins 2009 2 6/10/13 Jenkins 2009 Jenkins 2009 The Galacc Exncon Curve Extinction curves measure the difference in emitted and observed light. Traditionally measured by comparing two stars of the same spectral type. Galactic Extinction - empirically determined: -1 -1 <A(λ)/A(V)> = a(λ ) + b(λ )/RV (Cardelli et al. 1999) • Bump at 2175 Å (4.6 µm-1) • RV : Ratio of total to selective extinction in the V band • Mean value is RV = 3.1 (blue) • Low value: RV = 1.8 (green) (Udalski 2003) • High value: RV = 5.6-5.8 (red) (Cardelli et al.
    [Show full text]
  • 1 the Atmosphere of Pluto As Observed by New Horizons G
    The Atmosphere of Pluto as Observed by New Horizons G. Randall Gladstone,1,2* S. Alan Stern,3 Kimberly Ennico,4 Catherine B. Olkin,3 Harold A. Weaver,5 Leslie A. Young,3 Michael E. Summers,6 Darrell F. Strobel,7 David P. Hinson,8 Joshua A. Kammer,3 Alex H. Parker,3 Andrew J. Steffl,3 Ivan R. Linscott,9 Joel Wm. Parker,3 Andrew F. Cheng,5 David C. Slater,1† Maarten H. Versteeg,1 Thomas K. Greathouse,1 Kurt D. Retherford,1,2 Henry Throop,7 Nathaniel J. Cunningham,10 William W. Woods,9 Kelsi N. Singer,3 Constantine C. C. Tsang,3 Rebecca Schindhelm,3 Carey M. Lisse,5 Michael L. Wong,11 Yuk L. Yung,11 Xun Zhu,5 Werner Curdt,12 Panayotis Lavvas,13 Eliot F. Young,3 G. Leonard Tyler,9 and the New Horizons Science Team 1Southwest Research Institute, San Antonio, TX 78238, USA 2University of Texas at San Antonio, San Antonio, TX 78249, USA 3Southwest Research Institute, Boulder, CO 80302, USA 4National Aeronautics and Space Administration, Ames Research Center, Space Science Division, Moffett Field, CA 94035, USA 5The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA 6George Mason University, Fairfax, VA 22030, USA 7The Johns Hopkins University, Baltimore, MD 21218, USA 8Search for Extraterrestrial Intelligence Institute, Mountain View, CA 94043, USA 9Stanford University, Stanford, CA 94305, USA 10Nebraska Wesleyan University, Lincoln, NE 68504 11California Institute of Technology, Pasadena, CA 91125, USA 12Max-Planck-Institut für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany 13Groupe de Spectroscopie Moléculaire et Atmosphérique, Université Reims Champagne-Ardenne, 51687 Reims, France *To whom correspondence should be addressed.
    [Show full text]
  • Diffusion-Limited Escape/ the Atmospheric Hydrogen Budget/ Hydrodynamic Escape
    41st Saas-Fee Course From Planets to Life 3-9 April 2011 Lecture 6--Hydrogen escape, Part 2 Diffusion-limited escape/ The atmospheric hydrogen budget/ Hydrodynamic escape J. F. Kasting Diffusion-limited escape • On Earth, hydrogen escape is limited by diffusion through the homopause • Escape rate is given by (Walker, 1977*) esc(H) bi ftot/Ha where bi = binary diffusion parameter for H (or H2) in air Ha = atmospheric (pressure) scale height ftot = total hydrogen mixing ratio in the stratosphere *J.C.G. Walker, Evolution of the Atmosphere (1977) • Numerically 19 -1 -1 b i 1.810 cm s (avg. of H and H2 in air) 5 Ha = kT/mg 6.410 cm so 13 -2 -1 esc (H) 2.510 ftot (H) (molecules cm s ) Total hydrogen mixing ratio • In the stratosphere, hydrogen interconverts between various chemical forms • Rate of upward diffusion of hydrogen is determined by the total hydrogen mixing ratio ftot(H) = f(H) + 2 f(H2) + 2 f(H2O) + 4 f(CH4) + … • ftot(H) is nearly constant from the tropopause up to the homopause (i.e., 10-100 km) Total hydrogen mixing ratio Homopause Tropopause Diffusion-limited escape • Let’s put in some numbers. In the lower stratosphere −6 f(H2O) 3-5 ppmv = (3-5)10 −6 f(CH4 ) = 1.6 ppmv = 1.6 10 • Thus −6 −6 ftot (H) = 2 (310 ) + 4 (1.6 10 ) 1.210−5 so the diffusion-limited escape rate is 13 −5 8 -2 -1 esc (H) 2.510 (1.210 ) = 310 cm s Hydrogen budget on the early Earth • For the early earth, we can estimate the atmospheric H2 mixing ratio by balancing volcanic outgassing of H2 (and other reduced gases) with the diffusion-limited escape
    [Show full text]
  • Introduction to Chemistry
    Introduction to Chemistry Author: Tracy Poulsen Digital Proofer Supported by CK-12 Foundation CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook Introduction to Chem... materials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based Authored by Tracy Poulsen collaborative model termed the “FlexBook,” CK-12 intends to pioneer the generation and 8.5" x 11.0" (21.59 x 27.94 cm) distribution of high-quality educational content that will serve both as core text as well as provide Black & White on White paper an adaptive environment for learning. 250 pages ISBN-13: 9781478298601 Copyright © 2010, CK-12 Foundation, www.ck12.org ISBN-10: 147829860X Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made Please carefully review your Digital Proof download for formatting, available to Users in accordance with the Creative Commons Attribution/Non-Commercial/Share grammar, and design issues that may need to be corrected. Alike 3.0 Unported (CC-by-NC-SA) License (http://creativecommons.org/licenses/by-nc- sa/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), We recommend that you review your book three times, with each time focusing on a different aspect. which is incorporated herein by this reference. Specific details can be found at http://about.ck12.org/terms. Check the format, including headers, footers, page 1 numbers, spacing, table of contents, and index. 2 Review any images or graphics and captions if applicable.
    [Show full text]
  • The Longevity of Water Ice on Ganymedes and Europas Around Migrated Giant Planets
    The Astrophysical Journal, 839:32 (9pp), 2017 April 10 https://doi.org/10.3847/1538-4357/aa67ea © 2017. The American Astronomical Society. All rights reserved. The Longevity of Water Ice on Ganymedes and Europas around Migrated Giant Planets Owen R. Lehmer1, David C. Catling1, and Kevin J. Zahnle2 1 Dept. of Earth and Space Sciences/Astrobiology Program, University of Washington, Seattle, WA, USA; [email protected] 2 NASA Ames Research Center, Moffett Field, CA, USA Received 2017 February 17; revised 2017 March 14; accepted 2017 March 18; published 2017 April 11 Abstract The gas giant planets in the Solar System have a retinue of icy moons, and we expect giant exoplanets to have similar satellite systems. If a Jupiter-like planet were to migrate toward its parent star the icy moons orbiting it would evaporate, creating atmospheres and possible habitable surface oceans. Here, we examine how long the surface ice and possible oceans would last before being hydrodynamically lost to space. The hydrodynamic loss rate from the moons is determined, in large part, by the stellar flux available for absorption, which increases as the giant planet and icy moons migrate closer to the star. At some planet–star distance the stellar flux incident on the icy moons becomes so great that they enter a runaway greenhouse state. This runaway greenhouse state rapidly transfers all available surface water to the atmosphere as vapor, where it is easily lost from the small moons. However, for icy moons of Ganymede’s size around a Sun-like star we found that surface water (either ice or liquid) can persist indefinitely outside the runaway greenhouse orbital distance.
    [Show full text]
  • Atmospheric Escape and the Evolution of Close-In Exoplanets
    Atmospheric Escape and the Evolution of Close-in Exoplanets James E. Owen Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK; email: [email protected] Xxxx. Xxx. Xxx. Xxx. YYYY. AA:1{26 Keywords https://doi.org/10.1146/((please add atmospheric evolution, exoplanets, exoplanet composition article doi)) Copyright c YYYY by Annual Reviews. Abstract All rights reserved Exoplanets with substantial Hydrogen/Helium atmospheres have been discovered in abundance, many residing extremely close to their par- ent stars. The extreme irradiation levels these atmospheres experience causes them to undergo hydrodynamic atmospheric escape. Ongoing atmospheric escape has been observed to be occurring in a few nearby exoplanet systems through transit spectroscopy both for hot Jupiters and lower-mass super-Earths/mini-Neptunes. Detailed hydrodynamic calculations that incorporate radiative transfer and ionization chemistry are now common in one-dimensional models, and multi-dimensional calculations that incorporate magnetic-fields and interactions with the arXiv:1807.07609v3 [astro-ph.EP] 6 Jun 2019 interstellar environment are cutting edge. However, there remains very limited comparison between simulations and observations. While hot Jupiters experience atmospheric escape, the mass-loss rates are not high enough to affect their evolution. However, for lower mass planets at- mospheric escape drives and controls their evolution, sculpting the ex- oplanet population we observe today. 1 Contents 1.
    [Show full text]
  • Dependence of the Onset of the Runaway Greenhouse Effect on the Latitudinal Surface Water Distribution of Earth-Like Planets
    Dependence of the onset of the runaway greenhouse effect on the latitudinal surface water distribution of Earth-like planets T. Kodama1,2, A. Nitta1*, H. Genda3, Y. Takao1†, R. O’ishi2, A. Abe-Ouchi2, and Y. Abe1 1 Department of Earth and Planetary Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo, 113-0033, Tokyo, Japan 2 Center for Earth Surface System Dynamics, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba, 277-8568, Japan 3 Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan Corresponding author: Takanori Kodama ([email protected]) (Current addresses) *Tokyo Marine & Nichido Fire Insurance Co., Ltd. †International Affairs Department, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo, 152-0033, Japan Key Points: • The onset of the runaway greenhouse effect depends strongly on surface water distribution. • The runaway threshold increases as the surface water distribution retreats toward higher latitudes outside the Hadley circulation. • The lower the water amount on a terrestrial planet, the longer the planet remains in habitable condition. 1 Abstract Liquid water is one of the most important materials affecting the climate and habitability of a terrestrial planet. Liquid water vaporizes entirely when planets receive insolation above a certain critical value, which is called the runaway greenhouse threshold. This threshold forms the inner most limit of the habitable zone. Here, we investigate the effects of the distribution of surface water on the runaway greenhouse threshold for Earth-sized planets using a three- dimensional dynamic atmosphere model.
    [Show full text]
  • The Water Molecule
    Seawater Chemistry: Key Ideas Water is a polar molecule with the remarkable ability to dissolve more substances than any other natural solvent. Salinity is the measure of dissolved inorganic solids in water. The most abundant ions dissolved in seawater are chloride, sodium, sulfate, and magnesium. The ocean is in steady state (approx. equilibrium). Water density is greatly affected by temperature and salinity Light and sound travel differently in water than they do in air. Oxygen and carbon dioxide are the most important dissolved gases. 1 The Water Molecule Water is a polar molecule with a positive and a negative side. 2 1 Water Molecule Asymmetry of a water molecule and distribution of electrons result in a dipole structure with the oxygen end of the molecule negatively charged and the hydrogen end of the molecule positively charged. 3 The Water Molecule Dipole structure of water molecule produces an electrostatic bond (hydrogen bond) between water molecules. Hydrogen bonds form when the positive end of one water molecule bonds to the negative end of another water molecule. 4 2 Figure 4.1 5 The Dissolving Power of Water As solid sodium chloride dissolves, the positive and negative ions are attracted to the positive and negative ends of the polar water molecules. 6 3 Formation of Hydrated Ions Water dissolves salts by surrounding the atoms in the salt crystal and neutralizing the ionic bond holding the atoms together. 7 Important Property of Water: Heat Capacity Amount of heat to raise T of 1 g by 1oC Water has high heat capacity - 1 calorie Rocks and minerals have low HC ~ 0.2 cal.
    [Show full text]
  • The Transition from Primary to Secondary Atmospheres on Rocky Exoplanets
    50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1855.pdf THE TRANSITION FROM PRIMARY TO SECONDARY ATMOSPHERES ON ROCKY EXOPLANETS. M. N. Barnett1 and E. S. Kite1, 1The University of Chicago, Department of Geophysical Sciences. ([email protected]) Introduction: How massive are rocky-exoplanet hydrodynamic escape is illustrated below in Figure 1. atmospheres? For how long do they persist? These ques- tions are compelling in part because an atmosphere is necessary for surface life. Magma oceans on rocky ex- oplanets are significant reservoirs of volatiles, and could potentially assist a planet in maintaining its secondary atmosphere [1,2]. We are modeling the atmospheric evolution of R ≲ 2 REarth exoplanets by combining a magma ocean source model with hydrodynamic escape. This work will go be- yond [2] as we consider generalized volatile outgassing, various starting planetary models (varying distance from the star, and the mass of initial “primary” atmos- phere accreted from the nebula), atmospheric condi- tions, and magma ocean conditions, as well as incorpo- rating solid rock outgassing after magma ocean solidifi- cation. Through this, we aim to predict the mass and lon- Figure 1: Key processes of our magma ocean and hydrody- gevity of secondary atmospheres for various sized rocky namic escape model are shown above. The green circles exoplanets around different stellar type stars and a range marked with a V indicate volatiles that are outgassed from the of orbital periods. We also aim to identify which planet solidifying magma and accumulate in the atmosphere. These volatiles are lost from the exoplanet’s atmosphere through hy- sizes, orbital separations, and stellar host star types are drodynamic escape aided by outflow of hydrogen, which is de- most conducive to maintaining a planet’s secondary at- rived from the nebula.
    [Show full text]